A synthetic resin stopper can solve the problems associated with natural cork, such as corky odor and bacterial contamination, can be appropriately controlled in hardness and specific gravity, and has sufficient sealing property and pull-out property, and thus a synthetic resin stopper is being widely used in recent years as a substitute of a cork stopper.
As the synthetic resin stopper, there are proposed ones containing a foamed material obtained by foaming a composition containing a hydrogenated product of a block copolymer having a polymer block mainly containing a vinyl aromatic compound and a polymer block mainly containing a conjugated diene.
For example, Patent Document 1 discloses a container stopper containing, in at least a part that is brought into contact with an inner surface of a container mouth, an elastic material that is a independent cell type foamed material having an expansion ratio of from 1.01 to 4 times, containing a hydrogenated product of a block copolymer of an alkenyl aromatic hydrocarbon and a conjugated diene, or a mixture of the hydrogenated block copolymer and a polyolefin. It is stated that the container stopper has compression stress-strain characteristics that is close to those of wet cork, and is excellent in airtightness and gas barrier property, as compared to natural cork.
Patent Document 2 discloses a synthetic bottle stopper containing a foamed material of a thermoplastic elastomer composition containing a thermoplastic block copolymer, a foaming agent, and optionally a plasticizer, in which the composition further contains a branched polyolefin having a melt flow index of from 0.05 to 400. There is further disclosed that the thermoplastic block copolymer contains a glassy polymer block containing an aromatic vinyl monomer, and an elastic polymer block containing a conjugated diene.
There is stated as a characteristic feature that the synthetic bottle stopper disclosed in Patent Document 2 contains no oil as a plasticizer, and the suitable density thereof provides excellent stopper insertion and pull-out property with physical properties similar to cork.
Patent Document 3 discloses a molded closure for liquid container, containing a thermoplastic elastomer and a foaming agent. There is disclosed that upon inserting the molded closure into a container, the molded closure inhibits substantial permeation of oxygen into the container, does not absorb oxygen from the content of the container, is taken out from the container with a corkscrew without substantial expansion, crash or collapse, prevents substantially the content of the container from being putrefied, enables horizontal placement of the container immediately after inserting the molded closure into the container, and maintains permanently a printed matter on the surface of the molded closure. As the thermoplastic elastomer, styrene block copolymers, such as a styrene-ethylene-butylene-styrene copolymer, a styrene-butadiene-styrene copolymer and a styrene-isoprene-styrene copolymer, are exemplified.
Patent Document 4 discloses a thermoplastic foamable resin composition containing 100 parts by mass of a resin composition, which is obtained by mixing from 50 to 200 parts by mass of a softening agent for nonaromatic rubber and from 5 to 50 parts by mass of a polyethylene resin polymerized with a metallocene catalyst with 100 parts by mass of a hydrogenated block copolymer obtained by hydrogenating a block copolymer having one or more polymer block A mainly containing an aromatic vinyl compound and one or more polymer block B mainly containing a conjugated diene compound, and from 0.5 to 10 parts by mass of a chemical foaming agent having a decomposition temperature of 160° C. or less, and a foamed material thereof. There is stated that the composition is excellent in foaming property and flexibility upon foaming, and can have a reduced weight with a high expansion ratio.